A magnetic recording device such as hard disk drive would have the following key components: a recording medium to store information, a writing head to produce localized magnetic fields for writing information and a read sensor to convert the magnetic field from the media to electrical signals.
Each magnetic bit in the current perpendicular magnetic recording (PMR) medium comprises several thermally stable magnetic grains. Further increase in the areal density requires a reduction in the grain size to retain the signal-to-noise ratio.
However, the thermal stability factor (KuV/kBT, where Ku is the magnetocrystalline anisotropy, V the magnetic grain volume, kB the Boltzmann constant and T the absolute temperature) of each magnetic grain should be >60.1,2 Therefore, the reduction in the magnetic grain size reduces the thermal stability as a result of super-paramagnetism.3 Super-paramagnetism is a phenomenon by virtue of which magnetization direction of smaller magnetic particle switches without any applied magnetic field due to ambient thermal energy. In this case thermal energy (kBT), becomes comparable to the anisotropy energy (KuV) and magnetizations thermally flip the direction, which undesirably causes random data corruption. Putting another way, the magnetic grains lose the data undesirably without any applied field. Thus decreasing grain size is not by itself a solution to increasing areal density.
An alternative to delay the superparamagnetism is by using a material with large in order to keep the magnetic grains thermally stable, and hence the thermally stable bits. Prior art writing heads currently have a limitation of the maximum writing field of 24 kOe, and are unable to switch high Ku mediums such as CoPt and FePt, since switching field is proportional to magnetocrystalline anisotropy. One of the key challenges for the realization of high Ku materials based media for industrial application is to reduce the switching field.4 
Exchange coupled composite (ECC) bilayer media5 or exchange spring media6 (independently proposed by Victora et al. and Suess et al., respectively) may be effective at reducing the switching field. Such medium consist of magnetically hard and soft regions within each magnetic grain, where soft region assists hard region to reduce the switching field by formation of domain wall at the opposite end of the hard\soft interface followed by domain wall propagation towards the interface (FIG. 1.). In addition to reducing the writing field, ECC media also enjoys other advantages over conventional perpendicular magnetic recording such as faster switching and insensitivity to wider range of easy axis distribution.
Furthermore it has been theoretically predicted and experimentally observed that multilayer media or “graded media” in which anisotropy varies substantially continuously along the film growth direction, may be more effective for switching field reduction than the bilayer ECC media.7,8 However, preparing graded media is extremely challenging task using conventional sputtering technique. An approach to fabricate Fe/FePt graded media has been reported where Fe was deposited at high temperature on FePt, and composite film was annealed at high temperature.9 In this case diffusion of Fe into FePt may produce graded media. However, major drawback of this technique is that the control of Fe diffusion is very difficult. Moreover, annealing may also induce grain growth in the lateral direction, which undesirably deteriorates the signal-to-noise ratio.
Another approach being used is introducing an impurity during sputtering into the layer to increase the magnetic softness of hard magnetic material. But this way there are different layers formed with reduced impurity content hence it is more of a multilayer medium rather than a continuously graded media.10 